linux-kernel - Re: [PATCH 4/5] lib/list_sort: Simplify and remove MAX_LIST_LENGTH

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Date:   Thu, 14 Mar 2019 11:10:41 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     George Spelvin <lkml@....org>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Abramov <st5pub@...dex.ru>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Daniel Wagner <daniel.wagner@...mens.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Don Mullis <don.mullis@...il.com>,
        Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH 4/5] lib/list_sort: Simplify and remove
 MAX_LIST_LENGTH_BITS

On Tue, Mar 05, 2019 at 03:06:44AM +0000, George Spelvin wrote:
> Rather than a fixed-size array of pending sorted runs, use the ->prev
> links to keep track of things.  This reduces stack usage, eliminates
> some ugly overflow handling, and reduces the code size.
> 
> Also:
> * merge() no longer needs to handle NULL inputs, so simplify.
> * The same applies to merge_and_restore_back_links(), which is renamed
>   to the less ponderous merge_final().  (It's a static helper function,
>   so we don't need a super-descriptive name; comments will do.)
> 
> x86-64 code size 1086 -> 740 bytes (-346)

> +	do {
> +		size_t bit;
>  		struct list_head *cur = list;
> +
> +		/* Extract the head of "list" as a single-element list "cur" */
>  		list = list->next;
>  		cur->next = NULL;
>  
> +		/* Do merges corresponding to set lsbits in count */

> +		for (bit = 1; count & bit; bit <<= 1) {
> +			cur = merge(priv, (cmp_func)cmp, pending, cur);
> +			pending = pending->prev;  /* Untouched by merge() */
>  		}

Wouldn't be it the same to

	bit = ffz(count);
	while (bit--) {
		...
	}
?

Though I dunno which one is generating better code.

> +		/* And place the result at the head of "pending" */
> +		cur->prev = pending;
> +		pending = cur;
> +		count++;
> +	} while (list->next);
> +
> +	/* Now merge together last element with all pending lists */
> +	while (pending->prev) {
> +		list = merge(priv, (cmp_func)cmp, pending, list);
> +		pending = pending->prev;
>  	}

-- 
With Best Regards,
Andy Shevchenko